Can i use an aquarium R.O

What is wrong with your tap water
Where is the pressure/storage tank
50 GPD without a storage tank you will be standing there holding the hose , drip drip drip drip/

 

for a DWC grow or coco or what?

 

Yeah that one looks like it will work. Watch out for ones that 'add flavor' back in. You want ones like you found that claim zero tds, though its never really zero, still good enough.

 

check with ph pen first and adjust the nute accordingly is the way

good luck

 

Most hydroponic growers have issues including me with sodium lock out even if you are using 100% RO water.
Allot of Hydroponic nutrients have high amounts of sodium in it.

RO systems require 42 psi minimum input to operate anywhere near what is advertise GPD
I have a 5 stage with pressure pump
RO systems operate better with more back psi of the out put end of the RO system.
That is why Pressure tanks work the best.
How many gallons per day are you planning on using per day ?

 

IMO a RO system is pretty much worthless without a pressure tank.

Without getting your reservoir water nutrient solution tested you don't what is making up your PPMs at 350 or any PPM number it is
OK you have dissolved solids in your water/reservoir Allot of any one dissolved solid can give a high PPM reading. But you still don't know which dissolved solids it is.
electrical-conductivity-and-ph-guide-for-hydroponics-hla-6722.pdf (okstate.edu)

 

How do you know sodium is the issue ?

Personally I am not sure if you should be growing in hydroponics, no offence.
I want to see your grow be successful.
You have to have everything together, its a big learning curve were potting soil medium is not.
I have a nutrient photometer and the reagents because I can't afford 10 50 dollar tests several times a week..
I have ten DWC systems.
Think it over...

 

.Your trying to figure out your water nutrient and minerals you need and how much to use.
Plus you have to control your Reservoir temperature, plus be on the PH levels almost everyday.
And change the water in the reservoir every 7 days, and if you don't you will get sodium lock out.
Hydroponics has a big learning curve if you don't have someone if training you and your beginner.
Do you have a chiller?

 

I am not a coco grower..
I have seen my DWC systems use 4 to 6 gallon per day per sativa plant.
Why do you think I use so big of DWC systems be cause i leave home sometimes several days at a time